I really like Mortplayer Music, it allows setting particular folder(s) on either the internal or external memory or some of both.
And it allows playing your music either by playlist or by folder.
Music takes up the most room on my GT P3113, so putting it on my 32GB card makes sense.
I've used it for 2 years now. It does everything the way I like to do it. (I like playing my music by folder)
It's free and you can switch off the ads if you want to.
I also like Mortplayer Audio Books for listening to eBooks or other mp3 audio that you may have.
It remembers where you left off last and resumes.
For my pictures, I set the camera to use the external memory card, and use Quickpic for my picture management/viewing.
I've tried several, and this one seems to do all the good stuff.
good luck.